This invention relates to a polymeric bifocal lens production process in which a single-vision polymeric ophthalmic lens or lens blank has a second polymeric front surface cast onto it.
Throughout this specification, the term "single vision lens blank" means a finished or semi-finished lens blank having a nominally spherical convex front surface; the term "bifocal lens" means a lens which is a bifocal, a special bifocal with progressive power in the bifocal segment, or similar adaptation. It has long been recognised that the casting of a thermo-setting polymer to make a lens entails shrinkage during the polymerisation stage, typically resulting in a 15 percent reduction in volume of the lens. It has been proposed to use flexible moulds in order to accommodate this shrinkage and ensure that the moulds remain in contact with the lens during polymerisation to give a good optical surface. In U.S. Pat. No. 3248460 Naujokas has suggested that a new front surface of polymeric material can be cast onto a lens which has small surface defects attributable to shrinkage. He describes a method of casting a thin layer of polymeric material, having the same refractive index as that of the lens, onto the front surface of the lens to provide a means of recovering a lens having a major surface defect which is a more economical means than the normal polishing operation. In U.S. Pat. No. 4190621 Greshes has disclosed a process for manufacturing bifocal polymeric ophthalmic lenses by casting onto the front surface of a single vision lens. He casts a thin film onto the single vision lens to avoid further shrinkage problems during cure of that film. The patent describes the provision to a laboratory of a single vision lens which already contains the distance portion and the astigmatic correction together with the provision of a bifocal mould, which has the same radius of curvature as the front surface of the lens. The mould has a recessed portion so that two different thicknesses of film can be deposited on the lens. A small amount of catalysed monomer is deposited in the bifocal mould, held concave surface up. The single vision lens is then placed in the mould and spreads the liquid so that it completely fills the space between the lens and the mould. The lens is supported above the mound to hold the astigmatic correction at its proper angle, and to provide decentration of the bifocal portion. The assembly is then cured after which it is cooled and the lens is removed. Greshes does not disclose any particular method for preparing the single vision lens for the subsequent casting process, however, he does disclose that a major cause of lens failure was insufficient adhesion between the cast on portion and the original single vision base lens.
Published Australian patent application number PH 80566/87 discloses a lens production process in which a single vision polymeric ophthalmic (base) lens is prepared, and subsequently has a second polymeric front surface cast onto it. After removing any unwanted cylinder the base lens is prepared by roughening the surface of the lens by either chemical or physical means to assist the adhesion of the cast on layer. A concave bifocal mould is then chosen which has the same radius of curvature as the lens. Catalysed monomer is then injected onto the mould and evenly distributed by pressing down the base lens into the monomer. The base lens and mould are then held in the desired relative positions by a jig and the assembly is cured in an air oven or by ultra-violet radiation, The second polymeric front surface is preferably of the same refractive index as the base lens. However this method is not wholly satisfactory. For instance the monomer at the edge is subject to air inhibition and the polymer formed there is undercured. Further the cast on segment develops a gross optical defect along the top of the segment, this has been found to be due to shrinkage of the segment portion during and after curing. This shrinkage is also responsible for the assembly separating during the cure, i.e. the polymer separates from the mould.